Timing with new mech. advance

I have another question: I'm running only with the mechanical advance now, but I'm afraid that the total advance is too much. My car likes at least 15 of initial. I set the initial timing to 12 degrees but the car runs like crap. I need to know how much advance I get ONLY with the mechanical advance on. It's a stock 318 distributor with all mech. advance at 2500 rpm.